1. Field of the Invention
The present invention relates to a cooling structure for an internal combustion engine in which: a spacer is fitted inside a water jacket which is formed to surround peripheries of three or more cylinder bores arranged one after another on a cylinder row line of a cylinder block of the internal combustion engine; and a cooling condition of the cylinder bores is controlled by regulating a flow of cooling water in the water jacket by use of the spacer.
2. Description of the Related Art
Japanese Patent No. 3596438 has made publicly known such a cooling structure for an internal combustion engine in which: the heat transfer coefficient of the spacer fitted inside the water jacket is made different between the thrust/reverse-thrust sides of the cylinder bores (portions distant from the cylinder row line) and the inter-bore portions of the cylinder bores (portions close to the cylinder row line); and thereby, the cylinder bores are uniformly cooled throughout their whole peripheries.
Meanwhile, in a cylinder block in which three or more cylinder bores are arranged one after another along the cylinder row line, each of the two cylinder bores (end-portion cylinder bores) in the respective two end portions in the cylinder row line direction has only one adjacent cylinder bore. For this reason, each end-portion cylinder bore receives a smaller quantity of heat from its adjacent cylinder bore, and reaches a lower temperature. On the other hand, since each of the cylinder bores (intermediate cylinder bores) other than the end-portion cylinder bores has two adjacent cylinder bores, the intermediate cylinder bore receives a larger quantity of heat from the adjacent cylinder bores and reaches a higher temperature.
As described above, the temperature difference occurs between the end-portion cylinder bores and the intermediate cylinder bores. Even though the end-portion cylinder bores and the intermediate cylinder bores are thermally insulated equally by the spacer, all the temperatures of the cylinder bores cannot be made uniform. This leads to a problem that variations occur among the clearances between the pistons and the corresponding cylinder bores.